Method of making low thermal expansivity and high thermal conductivity substrate

ABSTRACT

A composite is provided which is adaptable to be a substrate for an electronic application. The composite comprises first material particles having a coefficient of thermal expansion in the range of about -20×10 -7  to about 50×10 -7  in/in/° C. Second material particles having a coefficient of thermal expansion in the range of about 100×10 -7  to about 200×10 -7  in/in/° C. are mixed with the first material particles. A bonding agent adheres the first and second material particles into a coherent composite having a coefficient of thermal expansion in the range of about 1×10 -7  to about 50×10 -7  in/in/° C.

This application is a division, of application Ser. No. 539,449, filedOct. 6, 1983 now U.S. Pat. No. 4,569,692.

While the invention is subject to a wide range of applications, it isespecially suited for use as a substrate for microelectronics andprinted circuits. This application relates to U.S. application Ser. No.863,655, filed May 15, 1986, a Reissue of U.S. Pat. No. 4,569,692. Theinvention is primarily concerned with bonding together materials havingdifferent thermal expansivities and thermal conductivities to form acoherent composite with desired properties tailored for individualapplications.

Low expansivity materials are widely used in the microelectronicindustry as substrate materials for individual packages, multi-devicehybrid circuit packages and printed circuit boards. The latterapplications are particularly useful when the coefficient of thermalexpansion of the printed circuit board is critical, i.e. when siliconchips or low expansivity leadless chip carriers are mounted on theboard.

In many instances, state of the art, low expansivity, ceramic andmetallic substrate materials may be replaced by relatively highexpansivity metals. Examples of these replacements are described in U.S.patent application Ser. Nos. 369,699, now U.S. Pat. No. 4,491,622,369,785 (now abandoned), 390,081 (now abandoned), 390,095 now U.S. Pat.No. 4,410,427, 398,497 now U.S. Pat. No. 4,480,262, 405,640 (nowabandoned), 454,409, 477,552 and 517,592 all of which are by the sameinventor, Sheldon H. Butt, and commonly assigned with the presentapplication. These patent applications generally describe the use ofhigh expansivity metallic materials in combination with appropriatelyselected high expansivity glasses or organic adherents. The mismatch incoefficient of thermal expansion between these combinations of materialsand silicon based microelectronic components is accommodated by mountingsystems such as adhesives or solders which prevent the development ofunacceptably high stresses upon the fragile and brittle siliconcomponents. However, for some applications it may not be feasible toprovide component mounting systems with silicon chips which canaccommodate the potential coefficient of thermal expansion mismatchstresses inherent with copper and copper alloy substrates. To betterappreciate the advantages of the materials of the present invention, afuller description of the limitations of the conventional substratematerials mentioned above follows.

Alumina ceramics are presently the most widely used substrate materials.There is a moderate mismatch between the coefficient of thermalexpansion of alumina and silicon which generally does not imposeunacceptable high stresses upon a silicon chip mounted on an aluminasubstrate and subjected to thermal cycling. The mismatch does not evenpose a particular problem when the chip sizes are quite large or whenthe chip is rigidly affixed to the substrate. The use of aluminaceramics as substrate material is particularly attractive since they aresomewhat less costly than most other presently available, lowexpansivity substrate materials. However, there are a number ofdrawbacks to these materials such as the thermal conductivity of alumina(Al₂ O₃) being poor, i.e. in the range of about 4 BTU/ft² /hr/°F. Also,because of present manufacturing capabilities, alumina substrate areasgreater than about 50 sq. in. are uncommon. Further, to add conductivecircuitry to the surface of alumina, thick film technology such asprinting circuitry upon the surface of the ceramic with a conductive inkis frequently used. The printed ink, usually precious metal based, issubsequently fired at elevated temperatures and consolidated intocontinuous conductors. Efforts to substitute copper based thick filminks for silver and gold based inks have been only partially successful.The inherent high cost of the materials and the tedious and costlyprocessing involved in generating circuitry upon the alumina substratesurface results in the alumina substrates being a relatively costlyarticle.

Beryllia ceramics, having a thermal conductivity of about 125 BTU/ft²/hr/°F. at 100° F., are used in the place of alumina ceramics inapplications requiring greater thermal conductivity than is availablewith alumina. The inherent high cost of beryllium results in thesesubstrates being very expensive. The same thick film printing and firingtechnology used with alumina is also required for beryllia. One furtherand unique disadvantage of using beryllia substrates is the toxicity ofberyllium. This problem may be solved by cautious handling and verycareful dust control. However, even with high material costs, highprocessing costs and toxicity control problems, beryllia substrates areoften used when enhanced thermal conductivity is required.

Molybdenum strip metal, having a thermal conductivity of about 820BTU/ft² /hr/°F. at 68° F., is sometimes used as a substrate material,particularly in hybrid circuit packages. Although expensive anddifficult to process, molybdenum does provide high thermal conductivityin association with a low coefficient of thermal expansion, i.e. 32×10⁻⁷in/in/°F. at 60° to 1060° F. A serious drawback to the use of molybdenumcomponents is the particular difficulty in processing because of itspoor oxidation resistance.

The use of special clad metals and substrate materials for lowexpansivity printed circuit board substrates has been described in someof the patent applications described hereinabove. These conceptsgenerally include cladding a high conductivity, high thermal expansivitycopper or copper alloy to a very low thermal expansivity nickel-ironalloy (such as INVAR). The resulting composite has a coefficient ofthermal expansion comparable to that of alumina and beryllia ceramics.The purpose of cladding with copper or copper alloy is to improve thenormally poor thermal conductivity of nickel-iron alloys. Because of thecharacteristics of the composite, the improved thermal conductivity isprimarily obtained in a plane extending longitudinally with the lengthand width of the substrate. However, the thermal conductivity throughthe thickness of the composite metal remains relatively poor because ofthe poor thermal conductivity through the nickel-iron alloy core. Thecost of the nickel-iron alloys and the relatively costly processinvolved in manufacturing clad metals results in these substratematerials being relatively expensive although less than comparableberyllia or molybdenum substrates.

Several other techniques have been taught by the prior art toaccommodate mismatched expansion coefficients as disclosed in U.S. Pat.Nos. 3,097,329 to Siemens and 4,320,412 to Hynes et al.

It is a problem underlying the present invention to reduce the mismatchin the coefficient of thermal expansion between microelectric componentsand substrates to which they are mounted while concurrently providingrelatively high thermal conductivity.

It is an advantage of the present invention to provide a composite whichobviates one or more of the limitations and disadvantages of thedescribed prior arrangements.

It is a further object of the present invention to provide a compositewhich substantially reduces the formation of stresses betweenmicroelectronics and printed circuit boards to which they are mounteddue to mismatch between the coefficients of thermal expansion.

It is a still further advantage of the present invention to provide acomposite which is adapted to be a printed circuit board having adequatestrength and thermal shock resistance.

It is another advantage of the present invention to provide a compositewhich may be either electrically conductive or electricallynon-conductive.

It is a yet further advantage of the present invention to provide acomposite which is relatively inexpensive to manufacture.

Accordingly, there has been provided a composite which is adaptable tobe a substrate for an electronic application. The composite comprisesfirst material particles having a coefficient of thermal expansion inthe range of about -20×10⁻⁷ to about 50×10⁻⁷ in/in/°C. Second materialparticles having a coefficient of thermal expansion in the range ofabout 100×10⁻⁷ to about 239×10⁻⁷ in/in/°C. are mixed with the firstmaterial particles. A bonding agent adheres the first and secondmaterial particles into a coherent composite having a coefficient ofthermal expansion in the range of about 1×10⁻⁷ to about 50×10⁻⁷in/in/°C. and preferably from about 20.3×10⁻⁷ to about 26.3×10⁻⁷in/in/°C.

The present invention is particularly directed to providing a range oflow expansivity, high thermal conductivity, economical compositematerials. They may be adapted to be substrates for semiconductordevices, hybrid packages or rigid printed circuit boards. Depending upontheir particular composition, they may be either electrically conductiveor electrically non-conductive. The basic concept involves mixingtogether several different materials to provide selected properties. Oneof the materials is selected from a readily available, very low ornegative thermal expansivity material. Such a material, as a class,generally exhibits poor thermal conductivity. Particles of this firstmaterial may be mixed with particles of a second highly thermalconductive material which characteristically and as a class exhibitshigher thermal expansivity. The second material is chosen to have ahigher thermal expansivity than required in the finished product so asto create a blended composite mass with properties selected and tailoredto the individual application. A third material bonds the mixture into acoherent mass having adequate strength and being resistant to thermalshock. Developing a mechanically strong, coherent composite mixture mustovercome the problem of adhering dissimilar materials together.

The low thermal expansivity particles may be chosen from anyconveniently available ceramic or glassy system. These may includematerials such as KRYPTONITE® which is a ceramic manufactured by OwensIllinois that has a negative coefficient of thermal expansion. Othermaterials include silica, borosilicate glasses and high silica ceramics.The preceding list is intended to be exemplary and it is also within thescope of the present invention to substitute other materials of aceramic or glassy nature which combine a low coefficient of thermalexpansion with other desirable characteristics.

The low thermal expansivity particles may also be produced from lowexpansivity metals or alloys. These include nickel-iron alloys whereinthe nickel is in the range of about 30 to 45% and the remainder isprimarily iron; iron, nickel and cobalt alloys; silicon or molybdenum.

The high thermal conductivity component of the composite mass mayconsist of commercially pure copper, high conductivity copper alloys,aluminum and its alloys, and other metals and metal alloys.

The specific choice of the high thermal conductivity material dependsupon the combination of properties desired in the final composite mass.For instance, it may be desirable to have a thermal conductivity in therange of about 65 to 230 BTU/ft² /hr/°F. The electrical conductivity isgenerally between about 30 to about 100% of the international annealedcopper standard (IACS). A degree of strength is required so that thematerial can be conveniently handled during processing. It may bedesirable for the material to have oxidation resistance, specified oxidecharacteristics, high temperature properties, desired magneticproperties, and other qualities depending upon the individualapplication.

The composite may be formed into a coherent mass of any desirable shapesuch as, for example, a strip or sheet. This may be accomplished bymixing the particles of one or more low thermal expansivity materialswith particles of one or more high thermal conductivity materials. Themixture is combined with a bonding agent or agents and formed into thedesired shape. This composite is then consolidated into a coherent massby the application of heat, pressure or a combination thereof so thatthe bonding material adheres to the various components of the mixture.The binding agent may include materials such as glasses, organicadhesives, metals like solder, and ceramics. It is also within the termsof the present invention to use other binding agents which do notinterfere with the final requirements of the composite.

Pretreatment of certain component particles may be necessary to promotetheir adhesion to the binding agent. For example, the low expansivitymaterials may not be wettable by the process step of materialconsolidation. These materials may require coating with a layer of metallike copper by any desirable process such as electroless plating. Otherpossibilities include coating the glass ceramic and metal materials withglass which is wettable by the bonding agent.

The composites of the present invention are broadly formed by mixingfirst material particles having a coefficient of thermal expansion inthe range of about -20×10⁻⁷ to about 50×10⁻⁷ in/in/°C. with secondmaterial particles having a coefficient of thermal expansion in therange of about 100×10⁻⁷ to about 239×10⁻⁷ in/in/°C. The mixture iscombined with a bonding agent which adheres the entire mass into acoherent composite having a coefficient of thermal expansion in therange of about 1×10⁻⁷ to about 50×10⁻⁷ in/in/°C. The desired range forthe coefficient of thermal expansion is chosen to be close to the valueof the coefficient of thermal expansion of silicon, i.e. 23.3×10⁻⁷in/in/°C. For many applications, the composite is formulated to have athermal expansion close to or the same as silicon. This prevents stressformation from thermal cycling when the composite is used as a substratewith a silicon device attached thereto. The above-described compositebroadly forms the present invention and examples of various formulationsare provided hereinbelow.

The first example is of a thermally and electrically conductivecomposite consisting of a low expansivity, electrically non-conductivecomponent in a continuous, predominantly copper matrix. Particles of lowexpansivity, borosilicate glass, sized between about 0.5 to about 5 mmin diameter, are coated with a metallic lead by a process of electrolessdeposition. To promote adhesion of the lead to the glass particles, itmay be desirable to chemically etch the surface of the glass particlesbefore deposition of the lead. The iron coated glass particles are thenmixed together with a binder of lead powder and high expansivity,different sized, copper particles. The proportion of lead and copper isadjusted so that the subsequently melted lead will fill the intersticesbetween the components of the mixture. The mixture is formed by pressureinto a sheet and consolidated by heating at a temperature in excess ofthe lead melting point, i.e. about 618° F., in an atmosphere reducing tocopper oxide and lead oxide. The removal of oxide films from themetallic particles is necessary to achieve complete wetting of thecopper by the lead. The result is a thermally and electricallyconductive composite consisting of a discontinuous phase of glassparticles in a matrix consisting of copper particles suspended in a leadmatrix. The lead was chosen as a bonding agent because of its relativelylow melting point and its low solubility in copper. This avoids thedegradation of the electrical and thermal conductivity of the copperparticles that could occur with a bonding agent such as a lead-tin alloyin which the tin could diffuse into the copper particle and reduce theelectrical and thermal conductivity.

Depending upon the specific constituents, the composite mass may requireexposure to a reducing atmosphere at a temperature above the meltingpoint of the lead to remove oxide films from the metallic particles andachieve substantially complete wetting of the copper by the lead.

Accelerated removal of the oxide films from the copper particles mayalso be achieved by utilizing a lead alloy bonding material whichcontains an element strongly reducing to copper, i.e. an almostcompletely lead alloy with calcium. The bonding may also be acceleratedby precoating the copper particles with lead or a phosphate coating. Thelatter coating may be accomplished by applying a phosphoric acidsolution containing from about 3.5 grams/liter up to the solubilitylimit of sodium dichromate (Na₂ Cr₂ O₇ Ch₂ O) or potassium dichromate(K₂ Cr₂ O₇) or mixtures thereof to the copper alloy material. Otherexamples of this process are disclosed in U.S. Pat. No. 4,525,422 toButt et al. With this precoating, the oxide films on the surface of thecopper particles are replaced by polyphosphate films which then dissolveand react with the lead.

Another process step may include increasing the hardness of the lead bysuitable alloying means such as the addition of antimony.

After the composite sheet has been formed, additional processing may bedesirable in accordance with the use requirements. For example, formicroelectronic substrate applications, the surface of the sheet may bedressed by chemical or mechanical means so as to smooth, polish, etc. orremove excess constituents from the surface. The substrate surface maynow be in condition to bond thin continuous metal foils such as copperor nickel to its surface by attachment to the lead or lead alloy phase.

Another alternative is to apply a continuous layer of a relatively lowmelting temperature glass, as compared to the melting temperature of thecomponents of the composition, to the substrate surface. This may beaccomplished by bonding the low temperature glass to the exposed glassparticles within the substrate. Preferably, the surface of the compositeis chemically dressed prior to the addition of the low temperature glasslayer by partially removing the metallic phase at the substrate surface.This process step exposes a larger percentage of the glass surfaceparticles within the matrix for adherence to the low temperature glasscoating.

Another exemplary embodiment is similar to the first example but withthe low melting temperature bonding agent eliminated. The glass orceramic particles of the low expansivity material may be coated byelectroless deposition with copper and mixed with copper powder. Theresulting composite is compacted into the desired form and consolidatedby sintering in a reducing atmosphere. As above, the reducing atmosphereis required to substantially eliminate the metal oxides which mayprevent the wetting of the glass by the copper. This choice of glassesand/or ceramics is limited to those which do not melt at the temperaturerequired to sinter the copper particles together. In cooling thesintered copper particles from the process temperature, substantialinternal stresses will develop as a result of the mismatch in thecoefficient of thermal expansion of the glass particles and the copper.These stresses may be relieved by slow cooling so that the copper phasecreeps and partially relieves these stresses.

Other electroless metal deposits such as silver or copper may be coatedon the surface of the glass as an alternative to the copper in order toachieve higher bond strength. The coating material is preferablyselected to either diffuse into the copper phase in small quantities andhave a minimal effect upon the copper phase or to be relativelyinsoluble in copper.

The copper particles of this example may be substituted with relativelyhigh strength, high conductivity copper alloy particles. For example,copper-iron alloy particles may be mixed with pure copper particles sothat through diffusion during sintering, the entire phase becomes arelatively higher strength copper-iron alloy. Other possibilitiesinclude copper alloys such as copper-chromium, copper-zirconium andcopper-cobalt.

Another embodiment provides iron particles mixed with the pure copperparticles. This has the effect of reducing the average coefficient ofthermal expansion of the metallic phase. After sintering, the metallicphase will consist of a mixture of iron-rich particles and copper-richcopper-iron alloy particles. This mixture advantageously providesenhanced strength, relatively higher conductivity and substantialmagnetic permeability.

The further embodiment is of an electrically non-conductive, thermallyconductive composite. To form this composite, low expansivity glass orceramic particles are mixed with copper or copper alloy particles. Arelatively low temperature melting glass binder (below the meltingtemperature of the other particles) adheres the mixture into a coherentcomposite and fills the interstices between particles. The composite ismade electrically nonconductive by mixing enough electricalllynonconductive, low expansivity particles to create a discontinuousmatrix of the metallic particles. Substantial thermal conductivity maybe obtained with this composite by limiting the thickness of the glassbinder film separating the conductive particles. The improvedconductivity can be enhanced by maximizing the metallic particle sizesand by minimizing the amount of low melting temperature binding glass.

The next embodiment is of another thermally conductive, electricallynon-conductive composite. This example is substantially the same as theprevious example except that an organic polymer such as epoxy, polyimideor a thermosetting organic resin replaces the low temperature meltingglass set forth above. As in the previous example, the thermalconductivity can be enhanced by minimizing the amount of the polymerbinder and maximizing the metallic particle size.

The patents and patent applications set forth in this application areintended to be incorporated by reference herein.

It is apparent that there has been provided a low thermal expansivityand high thermal conductivity substrate which is adaptable to be asubstrate for an electronic application which fully satisifes theobjects, means, and advantages set forth hereinabove. While theinvention has been described in combination with the specificembodiments thereof, it is evident that many alternatives,modifications, and variations will be apparent to those skilled in theart in light of the foregoing description. Accordingly, it is intendedto embrace all such alternatives, modifications, and variations as fallwithin the spirit and broad scope of the appended claims.

I claim:
 1. The process of forming a substrate for an electronicapplication, comprising the steps of:providing first material particleshaving a coefficient of thermal expansion in the range of about -20×10⁻⁷to about 50×10⁻⁷ in/in°C.; providing second material particles having acoefficient of thermal expansion in the range of about 100×10⁻⁷ to about239×10⁻⁷ in/in/°C.; mixing said first material particles with saidsecond material particles; and bonding the mixture of said first andsecond material particles together with a bonding agent to form acoherent composite being a strong, resistant to thermal shock substratehaving a coefficient of thermal expansion substantially less than about100×10⁻⁷ in/in/°C., said bonding agent adhering to said first and secondmaterial particles while maintaining the individual characteristics ofthe first and second material particles.
 2. The process of claim 1including the step of selecting said first and second materials so thatsaid composite has a thermal conductivity in the range of about 65 toabout 230 BTU/ft/ft² /hr/°F.
 3. The process of claim 2 including thestep of selecting said first material of particles from the groupconsisting of silica, ceramics, glass, metals and metal alloys.
 4. Theprocess of claim 3 further including said first material being particlesof silica.
 5. The process of claim 3 further including said firstmaterial being particles of glass.
 6. The process of claim 3 furtherincluding said first material being particles of metal.
 7. The processof claim 3 further including said first material being particles ofmetal alloys.
 8. The process of claim 3 further including said firstmaterial being particles of ceramics.
 9. The process of claim 3including the step of selecting said second material from the groupconsisting of metal and metal alloys.
 10. The process of claim 9including the step of forming said composite into a strip.
 11. Theprocess of claim 10 further including the step of bonding a metallicfoil to at least one surface of the composite strip.
 12. The process ofclaim 10 including the step of coating a continuous layer of low meltingtemperature glass onto at least one surface of the composite strip. 13.The process of claim 12 further including the step of partially removingsaid second material from at least one surface of said composite stripto enhance the adherence of the glass coating to the first materialparticles.
 14. The process of claim 1 including the steps of:compactingthe first and second material particles and the bonding agent into amixture of a desired shape; and consolidating the mixture with heat. 15.The process of claim 14 further including the step of heating saidmixture in a reducing atmosphere to substantially eliminate metaloxides.
 16. The process of claim 15 further including the step ofcoating the first material particles with the bonding agent prior tomixing the first and second material particles together.
 17. The processof claim 15 including the step of coating the first material particleswith the second material particles prior to mixing the first and secondmaterial particles together.
 18. The process of claim 2 including thestep of filling the interstices between the first and second materialparticles with a bonding agent.
 19. The process of claim 9 including thestep of selecting said bonding agent from the group consisting ofglasses, organic adhesives, ceramics and metals.
 20. The process ofclaim 19 further including said bonding agent being glass.
 21. Theprocess of claim 19 further including said bonding agent being anorganic adhesive.
 22. The process of claim 19 further including saidbonding agent being a ceramic.
 23. The process of claim 19 furtherincluding said bonding agent being a metal.